1. Field of the Invention
The present invention relates to printing plates with a water-developable photosensitive polymer composition having a high (or excellent) image-reproducibility and flexibility.
2. Description of the Prior Art
Photosensitive polymer printing plates having structure in which a photopolymerizable photosensitive polymer layer is formed on a metallic or plastic support have recently come into commercial use for relief-printing, lithography, and gravure printing. In using these printing plates, a negative or positive image-bearing film having transparent portions is placed closely against the photosensitive polymer layer, then irradiated with activating light. The portions of the photosensitive polymer layer corresponding to the transparent portions of the image-bearing film are thus photopolymerized. The unpolymerized portions of the photosensitive polymer layer are removed with an appropriate solvent. As a result, the desired relief image is formed on the support.
Photopolymerizable photosensitive polymer compositions are also used for numerous other applications in addition to the above-mentioned printing plates. Of these photosensitive Polymer compositions, those using partially saponified polyvinyl acetate as a base polymer have been proposed. Partially saponified polyvinyl acetate offers the advantage that unpolymerized portions thereof can be removed by neutral water.
Known examples of these photosensitive polymer compositions can be divided into two groups. The first group consists of those where the desired photosensitivity is provided by mixing the partially saponified polyvinyl acetate with a photopolymerizable unsaturated monomer mainly containly monofunctional acrylate or methacrylate having a hydroxyl group. These known photosensitive polymer compositions are disclosed in, for example, Japanese Examined Patent Publication (Kokoku) Nos. 46-39401, 50-3041, and 52-27561 and Japanese Unexamined Patent Publication (Kokai) Nos. 48-87903 and 50-27602.
The second group consists of those where the desired photosensitivity is provided by reacting a compound having an unsaturated group with the hydroxyl groups of the partially saponified polyvinyl acetate. These known photosensitive polymer compositions are disclosed in, for example, Japanese Examined Patent Publication Nos. 48-6962 and 49-5923 and Japanese Unexamined Patent Publication Nos. 48-55282, 48-65292, 48-66151, 50-30602, 50-45087, and 54-138090.
For instance, in the above-mentioned first group, it has been proposed to use monofunctional acrylate (i.e., an acrylic acid ester) or methacrylate (i.e., a methacrylic acid ester) having a hydroxyl group and one unsaturated group, such as .beta.-hydroxyethyl acrylate or methacrylate or such as .beta.-hydroxypropyl acrylate or methacrylate, as a main photopolymerizable monomer. These monofunctional unsaturated monomers have extremely good compatibility with partially saponified polyvinyl acetate. Therefore, these monofunctional unsaturated monomers function to improve the compatibility of polyfunctional acrylates or methacrylates having poor compatibility with partially saponified polyvinyl acetate and benzoin alkyl ether type photoinitiators having poor compatibility with partially saponified polyvinyl acetate.
However, to obtain satisfactory compatibility of the above-mentioned polyfunctional acrylates or methacrylates and the above-mentioned photoinitiators, a large amount of the above-mentioned monofunctional unsaturated monomers having a hydroxyl group or groups must be used. In such case, the crosslinking structure of the resultant polymer formed by photopolymerization would have a relatively low three-dimensional property due to the fact the polymer would be mainly formed by the monofunctional unsaturated monomers. This results in the disadvantages of insufficient water resistance of the photoinsolubilized portions and inability to obtain the desirable high image-reproducibility due to dissolution of minute image portions into water during water development. For these reasons, these photosensitive polymer compositions cannot easily be used for high-class printing plates requiring high image-reproducibility.
To eliminate these disadvantages, Japanese Unexamined Patent Publication No. 59-27602 proposes use of polyfunctional unsaturated monomers having two or more unsaturated groups and a number of hydroxyl groups more than the number of the unsaturated groups in the same molecule thereof. Since these polyfunctional unsaturated monomers have three or more hydroxyl groups, they have good compatibility with partially saponified polyvinyl acetate. Further, the density of the crosslinking structure obtained from these polyfunctional unsaturated monomers by photopolymerization is high. However, since an excess amount of the hydroxyl groups are bonded to the crosslinking structure, the resultant polymer has a strong hydrophilic property and poor water resistance. Thus, the photocured portions still partially dissolve in water during water development and these photosensitive polymers still cannot easily be used for high-class printing plates requiring high image-reproducibility.
As mentioned above, therefore, known photosensitive compositions where the photosensitivity is provided by mixing photopolymerizable unsaturated monomers with partially saponified polyvinyl acetate have insufficient water resistance of the photoinsolubilized portions and, therefore, cannot easily be used for printing plates requiring high image- reproducibility.
On the other hand, in the above-mentioned second group, where the desired photosensitivity is provided by incorporating an unsaturated group into partially saponified polyvinyl acetate, Japanese Examined Patent Publication No. 48-6962 and Japanese Unexamined Patent Publication Nos. 48-66151 and 50-45087 disclose incorporation of the unsaturated group into the partially saponified polyvinyl acetate by reacting the same with maleic acid or derivatives thereof. Furthermore, Japanese Unexamined Patent Publication No. 48-55282 discloses use of a cinnamic acid ester for incorporating the unsaturated group into the partially saponified polyvinyl acetate and Japanese Unexamined Patent Publication No. 48-65292 discloses use of an unsaturated lactone for incorporating the unsaturated group into the partially saponified polyvinyl acetate. However, in order to obtain satisfactory image-reproducibility only by the incorporation of the above-mentioned unsaturated groups into the partially saponified polyvinyl acetate, incorporation of a large amount of the unsaturated groups is required. The increase in the amount of the unsaturated groups incorporated results in a remarkable decrease in the amount of the hydroxyl groups, by which the water-solubility is afforded to the partially saponified polyvinyl acetate. This is because the compounds having the above-mentioned unsaturated group are incorporated into the partially saponified polyvinyl acetate by reaction thereof with the hydroxyl groups of the partially saponified polyvinyl acetate. The remarkable decrease in the amount of the hydroxyl groups in the partially saponified polyvinyl acetate causes a remarkable decrease in the water-development. Thus, the desirable balance of good image-reproducibility and good water-development cannot be accomplished by these prior compositions.
To eliminate these disadvantages, Japanese Examined Patent Publication No. 49-5923 proposes a composition of a photo-polymerizable acrylate together with a partially saponified polyvinyl acetate having the unsaturated group incorporated by reaction with N-methylol-acrylamide. Furthermore, Japanese Unexamined Patent Publication No. 54-138090 proposes a composition of a photopolymerizable acrylate together with a partially saponified polyvinyl acetate having the unsaturated group incorporated by reaction with an unsaturated aldehyde such as acrolein.
However, since the unsaturated group incorporated into the partially saponified polyvinyl acetate is neither an acryloyl group nor a methacryloyl group, its ability to copolymerize with the acryloyl group or the methacryloyl group of the photopolymerizable monomer is remarkably poor. Thus, the possibility or probability of chemically bonding the partially saponified polyvinyl acetate to the crosslinked structure formed by the polymerization of the copolymerizable monomer is very low. Accordingly, the expected improvement in the image reproducibility is not achieved.